Area (m², )

Acceleration (m/s², ft/s²)

Local speed of sound (m/s, ft/s)

Concentration (mass/volume, moles/volume)

Drag coefficient, defined different ways (dimensionless)

,

Heat capacity at constant pressure, volume (J/kg-K, Btu/- °F)

Diameter; , particle diameter (m, ft)

Hydraulic diameter (m, ft)

Mass diffusion coefficient (m²/s, ft²/s)

Total energy, activation energy (J, kJ, cal, Btu)

Mixture fraction (dimensionless)

Force vector (N, )

Drag force (N, )

Gravitational acceleration (m/s², ft/s²); standard values = 9.80665 m/s², 32.1740 ft/s²

Gr

Grashof number ratio of buoyancy forces to viscous forces (dimensionless)

Total enthalpy (energy/mass, energy/mole)

Heat transfer coefficient (W/m²-K, Btu/-h- °F)

Species enthalpy; , standard state enthalpy of formation (energy/mass, energy/mole)

Radiation intensity (energy per area of emitting surface per unit solid angle)

Mass flux; diffusion flux (kg/m²-s, /-s)

Equilibrium constant = forward rate constant/backward rate constant (units vary)

Kinetic energy per unit mass (J/kg, Btu/)

Reaction rate constant, for example, , , , (units vary)

Thermal conductivity (W/m-K, Btu/ft-h- °F)

Boltzmann constant ( J/molecule-K)

,

Mass transfer coefficient (units vary); also ,

Length scale (m, cm, ft, in)

Le

Lewis number ratio of thermal diffusivity to mass diffusivity (dimensionless)

Mass (g, kg, )

Mass flow rate (kg/s, /s)

Molecular weight (kg/kmol, /mol)

M

Mach number ratio of fluid velocity magnitude to local speed of sound (dimensionless)

Nu

Nusselt number dimensionless heat transfer or mass transfer coefficient (dimensionless); usually a function of other dimensionless groups

Pressure (Pa, atm, mm Hg, /)

Pe

Peclet number for heat transfer, and for mass transfer (dimensionless)

Pr

Prandtl number ratio of momentum diffusivity to thermal diffusivity (dimensionless)

Flow rate of enthalpy (W, Btu/h)

Heat flux (W/m², Btu/-h)

Gas-law constant (8.31447 J/kmol-K, 1.98588 Btu/mol- °F)

Radius (m, ft)

Reaction rate (units vary)

Ra

Rayleigh number ; measure of the strength of buoyancy-induced flow in natural (free) convection (dimensionless)

Re

Reynolds number ratio of inertial forces to viscous forces (dimensionless)

Total entropy (J/K, J/kmol-K, Btu/mol- °F)

Species entropy; , standard state entropy (J/kmol-K, Btu/mol- °F)

Sc

Schmidt number ratio of momentum diffusivity to mass diffusivity (dimensionless)

Mean rate-of-strain tensor (s^–1)

Temperature (K, °C, °R, °F)

Time (s)

Free-stream velocity (m/s, ft/s)

Velocity magnitude (m/s, ft/s); also written with directional subscripts (for example, , , , )

Volume (m³, )

Overall velocity vector (m/s, ft/s)

We

Weber number ratio of aerodynamic forces to surface tension forces (dimensionless)

Mole fraction (dimensionless)

Mass fraction (dimensionless)

Permeability, or flux per unit pressure difference (L/m²-h-atm, /-h-(/))

Thermal diffusivity (m²/s, /s)

Volume fraction (dimensionless)

Coefficient of thermal expansion (K^–1)

Porosity (dimensionless)

Ratio of specific heats, / (dimensionless)

Change in variable, final initial (for example, , , , , )

Delta function (units vary)

Emissivity (dimensionless)

Lennard-Jones energy parameter (J/molecule)

Turbulent dissipation rate (m²/s³, /s³)

Void fraction (dimensionless)

Effectiveness factor (dimensionless)

,

Rate exponents for reactants, products (dimensionless)

Radiation temperature (K)

Molecular mean free path (m, nm, ft)

Wavelength (m, nm, Å, ft)

Dynamic viscosity (cP, Pa-s, /ft-s)

Kinematic viscosity (m²/s, /s)

,

Stoichiometric coefficients for reactants, products (dimensionless)

Density (kg/m³, /)

Stefan-Boltzmann constant (5.67 W/m²-K⁴)

Surface tension (kg/m, dyn/cm, /ft)

Scattering coefficient (m^–1)

Stress tensor (Pa, /)

Shear stress (Pa, /)

Time scale, for example, , , (s)

Tortuosity, characteristic of pore structure (dimensionless)

Equivalence ratio (dimensionless)

Thiele modulus (dimensionless)

Angular velocity; , Mean rate of rotation tensor (s^–1)

Specific dissipation rate (s^–1)

Solid angle (degrees, radians, gradians)

Diffusion collision integral (dimensionless)